Metal oxide semiconductor field effect transistors (MOSFETs) have been traditionally used and widely applied in the semiconductor technologies. For deep sub-micron high performance MOS ULSI application, as suggested in the reference "B. Davari, in IEDM Tech. Dig., p. 555, 1996.", the dual poly gate CMOS technology (p+ poly gate for PMOSFET and n+ poly gate for NMOSFET) is necessary. However, as mentioned in"Y. Taur, et al., in IEDM Tech. Dig., p. 901, 1992.", the effect of boron penetration through the thin gate oxide into silicon substrate will degrade the device performance. There are several methods to suppress the boron penetration effects, such as (1) N.sub.2 O nitridation of thin gate oxide suggested in reference "E. Hasegawa, et al., in IEDM Tech. Dig., p. 327, 1995.", (2) the heavy nitrogen implantation (dosage.gtoreq.4E 15 cm-2) into poly-Si suggested in reference "S. Shimizu, et al., in IEDM Tech. Dig., p. 67, 1994.", and (3) the stacked-Si layer as gate material suggested in "S. L. Wu, et al., in IEDM Tech. Dig., p. 329, 1993", etc.
Although the heavy nitrogen implantation into poly-Si layer could effectively suppress the boron penetration effects, the sheet resistance of poly gate will be largely increased with increasing the nitrogen dosage for both n+ and p+ poly gates, especially for the nitrogen dosage larger than 4E15 cm.sup.-2. This is shown in the reference "S. Shimizu, et al., J. Appl. Phys., vol. 35, p. 802, 1996.".